1. Field of the Invention
The present invention concerns a rectifier able to operate with at least two separate ranges of alternating current supply voltage.
2. Description of the Prior Art
To be more precise, the present invention is directed to a rectifier able to adapt automatically to the voltage range including the voltage at which it is supplied with power. This type of rectifier is particularly useful when it is required to power a direct current device from an alternating current supply either to the American standard (110 V, 60 Hz) or to the French standard (220 V, 50 Hz), the voltage being liable to vary significantly about the reference voltage of 110 or 220 V. Other applications are also feasible.
FIGS. 1 and 2 show circuit diagrams of a prior art rectifier of this kind.
The rectifier 10 shown in these figures comprises a diode bridge D1-D4 of which an input (terminals 11 and 12) is connected to an alternating current voltage supply (not shown) and two reservoir capacitors C1 and C2 connected in series with each other and in parallel with an output of the diode bridge (terminals 13 and 14). The capacitors C1 and C2 are usually rated to suit the power output in a ratio of 2 .mu.F/W. Thus for a rectifier able to deliver a power or 200 W, the value of the capacitors C1 and C2 is 400 .mu.F. When the rectifier is designed to operate with ranges of supply voltage respectively centered on 110 V and 220 V, at a frequency in the order of 50 to 60 Hz, the ripple voltage across the capacitors C1 and C2 is less than 50 V.
A switch S is connected between an input terminal (terminal 12 here) and a serial connection point 15 between the capacitors C1 and C2. It is associated with a supply voltage range detector 17 and switch control means 16 responsive to the detector 17 and adapted to cause the switch S to be closed when the supply voltage is in a predetermined one of said supply ranges. In practise the switch S is a triac.
The entirely conventional operation of the circuit shown in FIGS. 1 and 2 will now be described.
In the prior art circuit shown in FIGS. 1 and 2, when the detector 17 detects a supply voltage in the second range, for example that centered on 220 V, a signal is sent to the control circuit 16 with the result that the switch is held open; when the detector 17 detects a supply voltage in the first supply range, that centered on 110 V, a signal is sent to the control circuit 16 with the result that the switch S is closed.
In the first case the current paths are as shown in FIG. 1. This figure shows the current I.sub.+ which flows during the positive half-cycle in continuous thin line. Note that during this half-cycle the diodes D1 and D2 conduct, the diodes D3 and D4 do not conduct and the current I.sub.+ charges the two capacitors C1 and C2. The current I.sub.- flowing during the negative half-cycle is shown in dashed thin line. Note that the diodes D3 and D4 conduct and that the diodes D1 and D2 do not conduct. The current charges the capacitors C1 and C2 equally. The currents I.sub.+ and I.sub.- flow in the same direction in the circuit consisting of the capacitors C1 and C2 connected in series.
When the detector 17 detects a voltage in the range centered on 110 V, the switch S is closed (FIG. 2). Note that the current I.sub.+ charges only the capacitor C2, the diode D1 being in a non-conducting state because the closing of the switch S has the result of applying to it the voltage across capacitor C1 which reverse biases it. During the negative half-cycle, the current I.sub.- charges only the capacitor C1, the diode D4 being in a non-conducting state because the closing of the switch S has the result of applying to it the voltage across capacitor C2 which reverse biases it.
In the circuit shown in FIG. 2, to close the switch S the control circuit 16 outputs a control current which is in practise applied to the trigger of the triac constituting the switch S. In some applications this control current is in the order of 20 mA, the control circuit typically consuming some 4 W. This is regarded as excessive.
Also, variations in the AC line voltage are sometimes considerable. Considering a rectifier able to operate in 110 V mode and in 220 V mode, this means that in practise, in 110 V mode, the rectifier must be able to operate in a range of voltages from 88 through 132 V, whereas in 220 V mode the rectifier must be able to operate in a range of voltages from 176 through 276 V.
Consequently, the power supply modules of these circuits include high-power resistors (rated at more than 5 W). These components are costly, which can rule out the marketing of rectifiers incorporating them on economic grounds.
It is therefore desirable to be able to reduce the consumption of the switch control circuit so that an inherent saving is achieved and to enable the use in the control circuit power supply circuit of significantly lower rated resistors (rated in the order of 2 W, for example), to achieve a further important saving.